Conventionally, an ISFET (Ion Sensitive FET) is known which is a field effect transistor (FET) in which a predetermined ion sensitive membrane is provided in a gate part. The ISFET can detect the change of gate potential generated by the reaction of the ion sensitive membrane to a predetermined ion.
Such an ISFET is known which functions as a pH sensor by employing tantalum oxide (Ta2O5) membrane or the like as the ion sensitive membrane so as to detect hydrogen ions in a liquid (for example, see the Patent Literature 1). The ISFET can detect various ions by controlling the material of the ion sensitive membrane suitably and is used largely as a sensor for measuring concentration or the like.
A device is proposed which measures automatically a pH of a liquid with the pH sensor mentioned above (for example, see the Patent Literature 2).
The automatic pH measuring device includes a pH sensor probe composed of a measurement pH sensor, a reference pH sensor, and a pseudo reference electrode at a predetermined position. The measurement pH sensor, the reference pH sensor and the pseudo reference electrode are soaked in a baseline solution and detect baseline potential so as to perform calibration. Subsequently, by absorbing a sample solution that is an object to be inspected into the pH sensor probe, the measurement pH sensor contacts the sample solution and detects the potential thereof so as to detect hydrogen ion concentration, whereby pH is measured.